1. Field of the Invention
The present invention relates generally to an image sensor and a process thereof, and more specifically to an image sensor and a process thereof that forms a color filter in an inter metal dielectric (IMD).
2. Description of the Prior Art
CMOS image sensors (CIS) are based on CMOS technologies, so the CMOS image sensors are compatible with the typical CMOS fabrication processes. They integrate additional signal processing logic elements on a same substrate, wherein it is possible to dispose sensor arrays. Thus, CMOS image sensors (CIS) are widely used.
An image sensor may be divided into function areas, such as a light sensing area and a peripheral electronic circuit area. The light sensing area comprises a plurality of photodiodes arranged in an array, and MOS transistors for sensing the light intensity, i.e. a reset transistor, a current source follower and a row selector. The peripheral electronic circuit area connects interconnects to external connections. A main function of the image sensor is to divide incident light beams into combinations of light beams of different wavelengths. The light is received by a plurality of imaging devices on the semiconductor substrate and transformed into digital signals of different intensities. For instance, an incident beam is divided into a combination of red, green and blue light and then received by corresponding photodiodes. Each photodiode transforms the light intensity into digital signals.
Some of the CIS systems use the Front Side Illumination (FSI) technology to form the pixels of a pixel array. In FSI CMOS image sensors, light is transmitted to a photo-sensing area through the front side of the pixel. This means that the incident light has to pass through dielectric layers, and metal layers before it reaches the photo-sensing area. The performances of the formed image sensors are enhanced through increasing the quantum efficiency (QE), reducing the cross talks between pixels, and reducing the dark current, which is achieved by improving the layout of the microlens, the color filters and the photo-sensing areas.